1. Field of the Invention
The present invention relates to a method and apparatus for detecting abnormal characteristic values of a plurality of products or lots sequentially manufactured in the same manufacturing line.
2. Description of the Related Art
In a first prior art abnormal characteristic value detecting method (see: JP-2001-67109-A), measured characteristic values depending upon lot numbers manufactured in the same manufacturing line have to fall within an allowable region. That is, when a measured characteristic value is outside the allowable region, a respective lot of this measured characteristic value is deemed to be defective, so that the respective lot is scrapped. Also, in order to decrease the number of scrapped lots, measured characteristic values are controlled to fall within a control region narrower than the allowable region. That is, when a measured characteristic value is within the allowable region but outside the control region, i.e., within an alarm region, an alarm signal is generated to carry out a countermeasure operation. Such a measured characteristic value is called an abnormal characteristic value. This will be explained later in detail.
In the above-described first prior art abnormal characteristic value detecting method, however, even if a successive-increase (or decrease) tendency is generated in the measured characteristic values, no alarm signal is generated so that a measured characteristic value would be outside the allowable region due to the delay of an advance countermeasure operation.
In a second prior art abnormal characteristic value detecting method, if a certain successive-decrease (or increase) tendency is generated even within the control region, an alarm signal is generated to prevent other measured characteristic values from being outside the allowable region. Even in this case, the last measured characteristic value of the tendency is called an abnormal characteristic value. This also will be explained later in detail.